This invention generally relates to gas analysis, and more particularly to the detection and measurement of nitric oxide in exhaled human breath.
At present, gaseous nitric oxide (NO) is most commonly measured by mixing a gas sample with ozone gas at low pressures. When a nitric oxide molecule reacts with an ozone (O3) molecule, it forms nitrogen dioxide (NO2) and oxygen (O2) and emits a photon in the process. This photon possesses a red or near-infrared wavelength. The concentration of nitric oxide in the gas sample is thus determined by measuring the intensity of those photons. However, red and near-infrared wavelengths are not detected efficiently by standard photodetecting devices such as photomultiplier tubes and photodiodes. Consequently, special photodetector devices that are more sensitive to red light must be used. These devices must be cooled to subambient levels to reduce background noise from thermal effects. These special devices and their cooling requirements add cost and complexity over that required to measure visible light.
In addition, an ozone-based nitric oxide gas detector requires a vacuum pump and a method for making ozone, which is typically a high-voltage electrical discharge. As a result, ozone-based detectors are generally bulky and complex, and require a significant amount of electricity to operate. The high voltage required to operate an ozone-based detector can pose a safety risk to the user and to those nearby. Government regulations restrict allowable ozone exposure, making it difficult to use ozone-based nitric oxide gas detectors in the workplace. Furthermore, ozone is a toxic gas, and it must be vented or destroyed after use. Because ozone is a pollutant, environmental regulations may prohibit venting the ozone in many areas, forcing the user of an ozone-based detector to destroy the ozone after use. Destruction of the ozone adds an additional step, and additional cost, to the nitric oxide measurement process.
Gaseous nitric oxide may also be detected by placing a gas sample in contact with an alkaline luminol solution containing hydrogen peroxide. As with the ozone-based method of detection, the chemical reaction between nitric oxide and the luminol solution results in the emission of photons. Unlike the ozone-based method of detection, these photons possess wavelengths in the more energetic end of the visible light spectrum. While the luminol-based method of detection overcomes some of the problems of ozone-based detection, it possesses drawbacks of its own. One drawback is toxicity of the chemicals used to detect ozone. Luminol, as well as bases which may be used to render the luminol solution alkaline, may be toxic if ingested or inhaled. Where human breath is to be measured, precautions must be taken to prevent such liquids, or fumes which may arise from them, from traveling through the measuring apparatus into the lungs of the person whose breath is being measured. Such precautions may cause the measurement process to be less efficient or in other ways interfere with the measurement of nitric oxide within exhaled human breath.
Another drawback is interference from the presence of carbon dioxide. When measuring atmospheric nitric oxide, carbon dioxide levels are typically too low (300-400 parts per million (PPM), which is 0.03-0.04 percent) to interfere with the measurement. However, carbon dioxide typically constitutes several percent of exhaled human or animal breath. This amount of carbon dioxide is orders of magnitude greater than the amount present in the atmosphere, and interferes with the detection and measurement of nitric oxide in human or animal breath when using a chemiluminescence-based detector. This interference primarily occurs in three ways. First, at a concentration of several percent, carbon dioxide reacts with the luminol solution to produce the same number of photons produced by the reaction of several parts per billion (PPB) of nitric oxide with luminol, tricking the detector into registering the presence of several PPB of nitric oxide which is not present in the sample. Second, carbon dioxide is known to react with a key intermediate in the nitric oxide/luminol reaction, ionic peroxynitrite (ONOOxe2x88x92). This reaction reduces the response of the luminol solution to nitric oxide, causing the detector to measure less nitric oxide than is actually present. Third, some gaseous carbon dioxide will dissolve in the alkaline luminol solution, changing its pH and thereby reducing the standing background signal of the luminol solution.
In one aspect of a preferred embodiment, the concentration of nitric oxide in a gas is determined by oxidizing NO to NO2, then measuring the concentration of NO2 in the gas, which is proportional to the concentration of NO and from which the concentration of NO is determined. Preferably, gaseous NO is converted to NO2 using chromium trioxide as a catalyst. In a particular embodiment, glass beads are coated with chromium trioxide, and the gas flows through the space between the beads. Contact between gaseous NO and the coated beads oxidizes the NO to form gaseous NO2.
In another aspect of a preferred embodiment, gas-permeable capillary membrane fibers transport a reagent solution through a chamber containing a gas to be analyzed. The capillary membrane fibers are constructed of a material porous enough, and are thin enough, to allow gaseous NO2 molecules to diffuse through and undergo a chemiluminescent reaction with the reagent within. The capillary membrane fibers are translucent, allowing photons emitted by the chemiluminescent reaction to pass through and be detected by a photodetector.
In another aspect of a preferred embodiment, gas is passed through a scrubber before entering the capillary membrane fibers. The scrubber removes carbon dioxide and ambient NO2 from the gas without substantially affecting the concentration of nitric oxide in the gas.
In another aspect of a preferred embodiment, gas to be analyzed is passed through a moisture exchanger to control its humidity. In another aspect, the moisture exchanger is located in a humidity-controller chamber. In a further aspect of a preferred embodiment, a humidity control unit including a water-absorbing material that is substantially saturated with water accepts dry gas at a pressure above atmospheric pressure, then discharges that gas at atmospheric pressure into the chamber, thereby controlling the humidity within the chamber.
In an aspect of a second preferred embodiment, nitric oxide is measured directly, without conversion to NO2 and without the use of a scrubber. The nitric oxide molecules penetrate the walls of capillary membrane fibers and undergo a chemiluminescent reaction with a reagent inside those fibers. In a further aspect of another preferred embodiment, the reagent is buffered at an alkaline pH and mixed with the enzyme carbonic anhydrase to reduce the measurement error that may be caused by the presence of carbon dioxide gas.
In an aspect of an alternate embodiment, a gas-permeable membrane within a plenum separates a first space containing gases to be measured from a second space containing a reagent. The membrane is thin enough and porous enough to enable gaseous nitric oxide molecules to pass through and undergo a chemiluminescent reaction with the reagent in the second space. The plenum is translucent, allowing photons emitted by the chemiluminescent reaction to pass through and be detected by a photodetector.
In an aspect of another alternate embodiment, the chemiluminescent reactant is not present in the reagent when the gaseous nitric oxide is exposed to the reagent. Rather, the chemiluminescent reactant is added in a second chamber, where the light produced by the chemiluminescent reaction is measured by a photodetector. Optionally, a carbonic anhydrase solution may be added in this second chamber to minimize the effect of carbon dioxide when nitric oxide is being measured in an environment containing a high concentration of carbon dioxide.